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Abstract

We have developed a new experimental setup based on optical Kerr gating in order to isolate either the transmitted or the scattered light going through an optically thick medium. This selectivity can be obtained by finely tuning the focusing of the different laser beams in the Kerr medium. We have developed an experimental setup. A Monte Carlo simulation scheme generates an accurate model of scattering processes taking into account the time of flight, the geometry of the Kerr gating and the polarization. We show that our experimental setup is capable of analyzing the transmitted light with optical densities up to OD = 9.7, and scattered light beyond OD = 347 in poly-disperse silica spheres in water (distribution centered on ~0.9µm radius) at λ = 550 nm. Strongly positive correlations are obtained with simulations.

Schematic representation of the sampling set up. The probe beam crosses the scattering sample S, and the iris I1 constrains the area of interest. The iris I2 defines the angle of collection of the scattered light. A convergent lens Lpr focuses both the ballistic and scattered light. The ballistic light follows a Gaussian propagation and focuses at the focal point F’. The scattered light follows a geometric propagation and the sample is imaged at plane (A’). The OKG plate is placed either at F’ or A’. P1 and P2 are two crossed polarizers.

Experimental and simulated temporal scattering diagrams in BPM configuration, for samples OD8, OD8.6 and OD9. Intensities and zero time delay are normalized to the experimental ballistic light peak. Scattered light is represented in (a) with a zoom on ballistic photons in (b).

Experimental and simulated temporal scattering diagrams in SLM configuration, for samples OD8.6, OD10.5, OD18 and OD42.7. Intensities are normalized to the maximum measured signal for each sample. Zero delay is the previously detected ballistic peak of OD8.6, not visible anymore in SLM.

Limits of detection of ballistic and/or scattered photons, relatively to the two different experimental configurations (BPM and SLM). The upper limits given for scattered light correspond to OD acquired with a signal / background ratio of ~1. The lower limit for scattered light corresponds to a scattered signal / ballistic signal ratio of 0.1. The upper limit for ballistic light corresponds to scattered signal / ballistic signal ratio of 2.